Please refer to the wiki for detailed documentation.
Building a debug version from command line
The build system is smart enough to automatically download and install all the parts of the Android SDK for you. If you cannot build, you can either try to fix your Android dev enviroment to fit the android/build.gradle requirements - or you can simply remove ANDROID_HOME, and all traces of your Android SDK from your PATH.
Building a debug version from Android Studio
Setup the Android Application to use two gradle aware make targets. You must set Package to be installed to "Do not deploy anything". Make sure you run assembleGithubDebug, then installGithubDebug.